<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>JavaScript基础用法</title>
</head>

<body>
    <script>
        // const 定义常量
        const PI = 3.1415926
        let r = prompt('请输入半径')
        // 模版字符串
        document.write(`圆的面积${PI * r * r}`)
    </script>
</body>

</html>